Exhaust system members of an automobile, including an exhaust manifold, an exhaust pipe, a converter case, and a muffler, are required to have high levels of oxidation resistance, thermal fatigue property, and high-temperature fatigue property (hereinafter these are collectively referred to as “heat resistance”). Upon initiation and stop of engine operation, exhaust system members are repeatedly heated and cooled. These members are restrained by their surrounding members, and thus their thermal expansion and contraction are restricted. As a result, the material itself experiences thermal strain, and this thermal strain causes fatigue phenomena. The thermal fatigue mentioned here represents this type of fatigue phenomenon. While an engine is under operation, the exhaust system members are heated and subjected to vibrations. These vibrations cause an accumulation of strain, also leading to fatigue phenomena. The high-temperature fatigue mentioned above represents this type of fatigue phenomenon. The former is low-cycle fatigue, whereas the latter is high-cycle fatigue. These are completely different types of fatigue phenomena.
As materials for such members requiring heat resistance as above, Cr-containing steels such as Type 429 containing Nb and Si (14Cr-0.9Si-0.4Nb system) are now widely used. However, improved performance of engines has increased the exhaust gas temperature to a level exceeding 900° C., making it impossible to fully achieve performance requirements, in particular, thermal fatigue property, with Type 429.
Some materials have been developed to address this problem, including Cr-containing steels that contain Nb and Si for an improved high temperature proof stress, SUS444 (19Cr-0.5Nb-2Mo) specified in JIS G4305, and ferritic stainless steels containing Nb, Mo, and W (e.g., see Japanese Unexamined Patent Application Publication No. 2004-018921). However, the recent terribly steep rise and fluctuation in the prices of Mo, W, and other rare metals have necessitated developing materials that can be made from inexpensive raw materials and have heat resistance comparable to that of the materials mentioned above.
An example of materials highly resistant to heat and containing no expensive elements such as Mo and W is that disclosed in International Publication No. WO 2003/004714, a ferritic stainless steel for members of automobile exhaust gas flow passages, which is based on a steel containing Cr at 10 to 20 mass % and further contains Nb at 0.50 mass % or less, Cu at 0.8 to 2.0 mass %, and V at 0.03 to 0.20 mass %. Another example is that disclosed in Japanese Unexamined Patent Application Publication No. 2006-117985, a ferritic stainless steel with excellent thermal fatigue property, which is based on a steel containing Cr at 10 to 20 mass % and further contains Ti at 0.05 to 0.30 mass %, Nb at 0.10 to 0.60 mass %, Cu at 0.8 to 2.0 mass %, and B at 0.0005 to 0.02 mass %. Yet another example is that disclosed in Japanese Unexamined Patent Application Publication No. 2000-297355, a ferritic stainless steel for automobile exhaust system components, which is based on a steel containing Cr at 15 to 25 mass % and further contains Cu at 1 to 3 mass %. These steels all contain Cu for improved thermal fatigue property.
Unfortunately, adding Cu as in WO '714, JP '985 and JP '355 admittedly improves thermal fatigue property but, on the other hand, significantly reduces oxidation resistance, ending up with reduced overall heat resistance. Worse yet, steels containing Cu may be somewhat lacking in thermal fatigue property during use under certain temperature conditions.
Some other patent publications have disclosed ferritic stainless steels containing Al for improved characteristics. An example is that disclosed in Japanese Unexamined Patent Application Publication No. 2008-285693, a ferritic stainless steel for automobile exhaust systems, which is based on a steel containing Cr at 13 to 25 mass % and further contains Ni at 0.5 mass % or less, V at 0.5 mass % or less, Nb at >0.5 to 1.0 mass %, Ti at 3×(C+N) to 0.25 mass %, and Al at 0.2 to 2.5 mass %. The addition of Al contributes to increased high-temperature strength. Another example is that disclosed in Japanese Unexamined Patent Application Publication No. 2001-316773, a heat-resistant ferritic stainless steel as a catalyst carrier, which is based on a steel containing Cr at 10 to 25 mass % and further contains Al at 1 to 2.5 mass % and Ti at 3×(C+N) to 20×(C+N). The added Al forms a coating of Al2O3 that provides excellent oxidation resistance. Yet another example is that disclosed in Japanese Unexamined Patent Application Publication No. 2005-187857, a heat-resistant ferritic stainless steel for hydroforming, which is based on a steel containing Cr at 6 to 20 mass % and further contains Ni at 2 mass % or less, O at 0.008 mass % or less, and any one or two or more of Ti, Nb, V, and Al at 1 mass % or less in total. The added Ti, Nb, V, and/or Al fixes C and N and forms a carbonitride to reduce the disadvantage of C and N, making the steel more formable.
Unfortunately, Al, when added to a steel with a low Si content as in JP '693, preferentially forms an oxide or a nitride and is solid-dissolved in a reduced amount, making the steel somewhat lacking in high-temperature strength. Also, Al, when contained in steel at a high content exceeding 1.0% as in JP '773, significantly reduces room-temperature workability and also causes reduces oxidation resistance rather than improving it because of a high binding affinity to oxygen. The steel disclosed in JP '857, which contains neither Cu nor Al or contains either only at a low content, is somewhat lacking in heat resistance.
However, our research revealed that, as with the steels disclosed in WO '714, JP '985 and JP '355 mentioned above, adding Cu for improved heat resistance admittedly improves thermal fatigue property but, on the other hand, significantly reduces oxidation resistance of the steel itself rather than improving it and often ends up with reduced overall heat resistance. Furthermore, we also found that steels containing Cu may be somewhat lacking in thermal fatigue property during use under certain temperature conditions, for example, conditions under which the maximum temperature is lower than the solid-dissolution temperature of ε-Cu.
Although JP '693 and JP '773 state that adding Al leads to great high-temperature strength and excellent oxidation resistance, our research has found that merely adding Al ends up with an insufficient effect and that the balance between the amount of Al and that of Si is important. Steels containing neither Cu nor Al or containing either only at a low content as in JP '857 are somewhat lacking in heat resistance.
The oxidation resistance of steel is usually assessed by an oxidation test in a dry and high-temperature atmosphere. However, an exhaust manifold and other exhaust system members are exposed to an oxidative atmosphere in practical use, and such an atmosphere contains a large amount of vapor. Thus, the existing oxidation tests cannot adequately assess the practical oxidation resistance of steel. As is clear from this fact, the oxidation resistance of steel should be assessed and improved in consideration of that in a water vapor atmosphere (hereinafter also referred to as “water vapor oxidation resistance”).
Thus, it could be helpful to develop a technique for producing steel without adding expensive elements such as Mo and W while preventing the oxidation resistance loss after the addition of Cu and improving the characteristics at temperatures tough for the steel (temperatures lower than the solid-dissolution temperature of ε-Cu) and thereby provide ferritic stainless steels having excellent levels of oxidation resistance (including water vapor oxidation resistance), thermal fatigue property, and high-temperature fatigue property.
Note that the expression “having excellent levels of oxidation resistance, thermal fatigue property, and high-temperature fatigue property” means that these characteristics of the steel are at least equivalent to those of SUS444. More specifically, this expression means the following: As for oxidation resistance, the oxidation resistance at 950° C. of the steel is at least equivalent to that of SUS444. As for thermal fatigue property, the resistance of the steel to the fatigue from thermal cycling in the temperature range of 100° C. to 850° C. is at least equivalent to that of SUS444. As for high-temperature fatigue property, the high-temperature fatigue property at 850° C. of the steel is at least equivalent to that of SUS444.